I guess Rust made the same reasoning than Python. For this kind of things, the PEP documents tend to be well based on experience and be a good guideline which even applies for other languages. Check the PEP 8 that I linked in my comment: although they recommend a very conservative limit of 80 (79 actually) it says that if it makes sense, 100 (99) can be used too. And that's from 2001.